This invention applies to the combustor section of gas turbine engines used in powerplants to generate electricity. More specifically, this invention relates to the structure that transfers hot combustion gases from a can-annular combustor to the inlet of a turbine.
In a typical can annular gas turbine combustor, a plurality of combustors is arranged in an annular array about the engine. The hot gases exiting the combustors are utilized to turn the turbine, which is coupled to a shaft that drives a generator for generating electricity. The hot gases are transferred from the combustor to the turbine by a transition duct. Due to the position of the combustors relative to the turbine inlet, the transition duct must change cross-sectional shape from a generally cylindrical shape at the combustor exit to a generally rectangular shape at the turbine inlet, as well as change radial position, since the combustors are typically mounted radially outboard of the turbine.
The combination of complex geometry changes as well as excessive temperatures seen by the transition duct create a harsh operating environment that can lead to premature repair and replacement of the transition ducts. To withstand the hot temperatures from the combustor gases, transition ducts are typically cooled, usually by air, either with internal cooling channels or impingement cooling. Catastrophic cracking has been seen in internally air-cooled transition ducts with excessive geometry changes that operate in this high temperature environment. Through extensive analysis, this cracking can be attributed to a variety of factors. Specifically, high steady stresses have been found in the region around the aft end of the transition duct where sharp geometry changes occur. In addition stress concentrations have been found that can be attributed to sharp corners where cooling holes intersect the internal cooling channels in the transition duct. Further complicating the high stress conditions are extreme temperature differences between components of the transition duct.
The present invention seeks to overcome the shortfalls described in the prior art and will now be described with particular reference to the accompanying drawings.